Optical components for microlithography or preforms for optical fibers are for instance made from synthetic quartz glass. In the manufacture of the quartz glass a so-called “soot body” of porous SiO2 soot is often obtained as an intermediate product. The soot body forms a layer of porous SiO2 soot, which may e.g. be present in the form of a solid cylinder, a hollow cylinder or as a SiO2 soot coating on a rod or tube of quartz glass or of another material. Such SiO2 layers are e.g. obtained by axial deposition of SiO2 soot according to the VAD (Vapor Axial Deposition) method or by deposition on the cylinder jacket surface of a carrier rotating about its longitudinal axis according to the OVD (Outside Vapor Deposition) method.
The formation of the SiO2 particles in the deposition of the SiO2 soot is normally carried out by hydrolysis or oxidation of a silicon-containing raw material, such as SiCl4, by means of an oxyhydrogen flame. Therefore, due to the manufacturing process the SiO2 soot body contains a great number of hydroxyl groups. These have an impact on the resistance of the quartz glass to short-wave UV radiation, they influence the optical transmission, and they change the refractive index of the quartz glass obtained therefrom.
Therefore, in the manufacture of quartz glass, particular attention is normally paid to the observation of a predetermined mean content of hydroxyl groups. Excessive hydroxyl groups must be removed prior to the vitrification of the SiO2 soot body. Chemical drying processes and thermal drying processes are known to remove hydroxyl groups.
Chemical drying is efficient and fast. It is based on substitution or reaction of the hydroxyl groups with a reagent, for instance chlorine. The synthetic quartz glass dried in this way contains, however, a certain amount of the drying reagent, for instance chlorine or fluorine, and it is known that this may deteriorate the resistance of the quartz glass to UV radiation.
Therefore, the hydroxyl groups must be thermally removed from the SiO2 soot body in applications where the soot body cannot be dried chemically. The thermal drying process is based on diffusion, resulting in a concentration gradient of hydroxyl groups between the inner portion of the drying SiO2 soot body and its free cylinder jacket surfaces.
In the case of quartz glass cylinders used for making lenses or other microlithographic components, a radial distribution of the hydroxyl groups that is as homogeneous as possible is however desired. Ideally, the hydroxyl group content is so uniform over the radius of the quartz glass cylinder that a subsequent and expensive homogenization of the quartz glass is not needed.
US 2002/007347 A1 discloses a method for doping a soot preform with fluorine. The document suggests that a porous SiO2 soot preform should be heated to a temperature above 1200° C. and that the preform should be exposed to a fluorine-containing atmosphere and that the treatment duration and the temperature should be set such that a predetermined radial profile of the fluorine concentration is obtained. As a kind of support, and while the soot body is built up, a barrier layer is produced in the form of a vitrified thin layer within the soot body. During the subsequent treatment in fluorine-containing atmosphere said barrier layer is to reduce the diffusion of fluorine into the inner portion of the soot body so as to ensure a distinct refractive index jump in this portion.
A method for thermally drying a SiO2 soot body in the course of the manufacture of a solid cylinder of quartz glass is known from WO 2006/082983 A2. It is suggested therein that a SiO2 soot body produced according to the VAD method should be presintered at a temperature around 1350° C. while maintaining a porous soot body structure, so that a mean soot density ranging from 0.3 g/cm3 to 0.8 g/cm3 is set. The presintered porous SiO2 soot body is then dried at a temperature of around 1250° C. under vacuum for a period of time of 20 to 100 hours and is subsequently vitrified at a high temperature of around 1750° C. under inert gas. Presintering and subsequent drying of the presintered porous quartz glass body result in a noticeable decrease in the mean hydroxyl group content and also in a homogeneous radial distribution of the hydroxyl group concentration in the finished solid cylinder of quartz glass.
US 2002/0073740 A1 discloses a method for manufacturing a preform having at least one fluorine doped region. A cylindrical soot body is prepared which is doped with fluorine in a doping process. In order to control the radial penetration of the fluorine in the soot body during the doping process it is suggested to generate at least one glassy barrier layer by densification of a thin soot layer.